This invention relates to a method and apparatus for preventing the generation of false color signals in a single-tube or two-tube type color television camera of the type in which at least two color component signals modulated spatially by striped color filters are demodulated by the technique of phase separation, through the utilization of correlation (vertical correlation) in successive lines of scan. More particularly, the invention relates to a method and apparatus for preventing the generation of false color signals in a camera so as to suppress color errors produced at the vertical edge portions of a reproduced image on a picture screen of a color television reproducing apparatus.
With a known color television camera designed so that the modulated color signals produced by spatially modulating at least two color components by striped color filters are demodulated by utilizing vertical correlation on the image, in view of the occurrence of color errors at the vertical edge portions of a reproduced image inherent to the system, it has been the practice such that the difference between a luminance signal and a signal produced by delaying the luminance signal one horizontal scan period is detected to produce a difference signal or a color error detection signal representing the positions of the vertical edge portions, and the gain of a color signal demodulation circuit is controlled by the difference signal, thereby correcting the color errors. FIG. 1 of the accompanying drawings is a block diagram of this type of known color television camera, showing a pick up tube 1 with striped filters, a preamplifier 2, a low-pass filter 3 for removing the modulated component due to the striped filters, to produce a luminance signal Y, a band-pass filter 4, a color signal demodulation circuit 5 for producing the color component signals or red component signal R and blue component signal B from the output of the band-pass filter 4, a delay circuit 6 for providing a delay of one horizontal scanning period, a .pi./2-radian phase shifter 7, a gain control circuit 8 for adjusting the amplitude of the output from the delay circuit 6 by the difference signal A, an adder 9 for producing the signal R, a subtractor 10 for producing the signal B, an encoder 11 for producing an NTSC standard signal from the luminance signal Y and the color component signals R and B, another delay circuit 12 for generating a signal produced by delaying the luminance signal Y by one horizontal scanning period, and a subtractor 13 for producing the difference between the signal from the delay circuit 12 and the luminance signal Y to generate the difference signal A.
In a color television camera of the type shown in FIG. 1, the vertical signal waveforms corresponding for example to a black and white pattern image comprising upper and lower black areas and a middle white area become as shown in FIG. 2. More specifically, FIG. 2 (a) shows the luminance signal Y, (b) the average of the signal R and (c) the average of the signal B. As shown in (d), a green component G appears in the white area of the vertical edge portion where the image pattern changes from black to white and a magenta component (R+B) appears in the black area of the vertical edge portion where the image pattern changes from white to black. Particularly, in the case of an image of a televised black and white object having a high contrast, these color errors are caused by the fact that the correction through the gain control of the color signal demodulation circuit by the difference signal A cannot satisfactorily follow or respond to the dynamic range of the object.
While the problem of incomplete correction of color errors in the above-mentioned prior art methods can be solved by, for example, a method proposed in U.S. Pat. No. 4,104,679 in which only when there is no vertical correlation in an image the color signals are prevented by the difference signal A and the image is made monochrome, this method, however, cannot always be considered expedient since all the colors are cancelled at the vertical edge portions of an image and the image is made extremely unnatural in the case of a reddish object such as the face of a man.